Method for forming barrier film

ABSTRACT

Barrier film with a high colorless transparency comprising a flexible plastic substrate. A barrier layer is formed on the surface and has a thickness ranging from approximately 50 to less than 200 Angstroms and is formed of a material selected from the group of aluminum oxide, tin oxide and yttrium oxide. An additional barrier layer formed of silicon dioxide may also be formed on the barrier layer.

This application is a continuation-in-part of application Ser. No.07/785,870 filed Nov. 1, 1991, now abandoned, which is continuation ofapplication Ser. No. 07/535,183, filed Jun. 8, 1990, now abandoned,which is a continuation-in-part of application Ser. No. 07/426,342 filedon Oct. 24, 1989, now abandoned.

This invention relates to a barrier film having high colorlesstransparency and a method for forming the same.

Barrier films have heretofore been provided. Typical coatings are of thetype disclosed in British patent specification 1,086,482. It isdisclosed therein that the preferred inorganic coatings are oxides ofsilicon and aluminum which can be deposited as transparent flexiblecoatings in a glassy state. Silicon monoxide or silicon dioxide arementioned as starting materials and aluminum oxide for the aluminumcoatings. This aluminum oxide is identified in Table V as being NortonAlundum #4186. This is not a pure aluminum oxide but a mixture ofaluminum oxide and silicon dioxide (Al₂O₃ in a SiO₂ binder). Zirconiumoxide is also mentioned as a starting material. However, this materialis not particularly suited for evaporation techniques because of itshigh melting temperature. Silver chloride which is also identified as astarting material is undesirable because it becomes hazy when depositedas a coating material. With respect to all of the starting materialsmentioned, they are deposited as a single layer. In coating operations,the roll speed is very slow at the rate of 3 inches per minute. Thesingle layer is also deposited to a thickness, as for example, 6000Angstroms which is very thick. It is pointed out that the minimumthickness is 0.02 microns (200 Å). Below this thickness the inorganicbarrier layer is ineffectual. The tables in the British patentspecification 1,086,482 disclose the barrier properties with respect tooxygen and helium, but do not address water permeability. In addition,in U.S. Pat. No. 4,702,963 there is disclosed a barrier film which ismade from silicon monoxide or a suboxide of SiO₂. Although this barrierfilm has good barrier properties, it has the undesirable feature that ithas an amber color. This amber color is objectionable in many packagingapplications because it obscures the true color of the product withinthe package. It has been found that when silicon dioxide is depositeddirectly on a film by electron beam evaporation no additional barrierproperties are provided by the silicon dioxide. There is therefore aneed for a new and improved barrier film which has a colorlesstransparency. In addition, there is a need to replace existingaluminized polyester and co-extruded polymeric films. Also, in view ofsolid waste disposal problems, there is a need to limit the use ofnon-recyclable plastic films. There is also a need to reduce the totalvolume of plastic waste which is non-recyclable by reducing thethickness and the number of plastic layers and by recycling the plasticfilm. Co-extruded plastic film structures are not easily recycledbecause of the complexity of the chemical structures in theco-extrudants. There is also a need to reduce the use of PVC and PVDC asbarrier materials in film in order to eliminate migration of un-reactedmonomers reaching food contents packaged within such barrier films.

In general, it is an object of the present invention to provide abarrier film having high colorless transparency and a method for makingthe same.

Another object of the invention is to provide a barrier film of theabove character having water white high colorless transparency.

Another object of the invention is to provide a barrier film of theabove character which has an overall reduced thickness.

Another object of the invention is to provide a barrier film of theabove character which can run efficiently in conventional cigarettepackaging machines.

Another object of the invention is to provide a barrier film of theabove character which has a thickness in the range of 70-80 gauge.

Another object of the invention is to provide a barrier film of theabove character which has decreased yield losses.

Another object of the invention is to provide a barrier film of theabove character which will not curl.

Another object of the invention is to provide a barrier film of theabove character which is a monolithic product that can be heat sealedonto itself.

Another object of the invention is to provide a barrier film of theabove character which has low friction.

Another object of the invention is to provide a barrier film of theabove character which is particularly suitable for tobacco products suchas cigarettes.

Another object of the invention is to provide a barrier film of theabove character which can be produced at a reduced cost.

Another object of the invention is to provide a barrier film of theabove character which will tear in a straight line.

Another object of the invention is to provide a barrier film of theabove character which does not require the use of aluminum.

Another object of the invention is to provide a barrier film of theabove character which does not require the use of co-extruded polymericfilms.

Another object of the invention is to provide a barrier film of theabove character which reduces the total volume of plastic required.

Another object of the invention is to provide a barrier film of theabove character which reduces the difficulty of recycling.

Another object of the invention is to provide a barrier film of theabove character which can be utilized for food packaging which can beused in microwave ovens.

Another object of the invention is to provide a barrier film of theabove character which uses a non-metal which can be utilized forpackaging food which can be used in a microwave unit and still betransparent with a long shelf life.

Another object of the invention is to provide a barrier film of theabove character in which PVDC need not be utilized as barrier materials.

Another object of the invention is to provide a barrier film of theabove character which can maintain a given moisture content for contentspackaged in the barrier film.

Another object of the invention is to provide a barrier film and methodin which the barrier film can be produced in a roll coater at highproduction speeds.

Another object of the invention is to provide a barrier film of theabove character which is provided with a heat seal layer so that it canbe readily utilized as a self-sealing packaging material.

Another objection of the invention is to provide a barrier film of theabove character which serves as an overwrap packaging film that willincrease shelf life of moisture and oxygen sensitive products.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments are set forthin detail in conjunction with the accompanying drawings.

FIG. 1 is a cross sectional view of a barrier film incorporating thepresent invention in which a barrier layer is provided on one side ofthe flexible plastic substrate.

FIG. 2 is a cross sectional view of a barrier film incorporating thepresent invention in which a barrier layer is provided on both sides ofthe flexible plastic substrate.

FIG. 3 is a cross sectional view similar to the one shown in FIG. 1 witha heat seal layer to facilitate use of the barrier film as a packagingmaterial.

FIG. 4 is a cross sectional view similar to FIG. 2 but showing the useof heat seal layers on both sides of the substrate to facilitate use ofthe barrier film in packaging materials.

FIG. 5 is a cross sectional view of a barrier film in which the barrieris provided by a single layer.

FIG. 6 is a graph showing water vapor transmission data using aluminumoxide and silicon dioxide layers.

FIG. 7 is a graph showing water vapor transmission data for tin oxideand silicon dioxide layers.

FIG. 8 is a graph showing water vapor transmission data using yttriumoxide and silicon dioxide layers.

FIGS. 9 and 10 are cross-sectional views showing two pre-laminationconstructions incorporating the present invention for a packaging film.

FIG. 11 is a cross-sectional view showing the packaging filmpost-lamination.

FIG. 12 is a cross-sectional view of a heat seal coated packaging film.

FIG. 13 is a cross-sectional view of a barrier-type film constructionwhich is heat sealable onto itself and on which the two layers of abarrier coating are provided facing each other.

FIG. 14 is a graph showing water vapor transmission rates for singlelayer aluminum oxide (AL₂O₃) on 70 gauge polypropylene.

FIG. 15 is a cross-sectional view of another embodiment of a barrierfilm incorporating the present invention which is in the form of amonolithic product.

FIG. 16 is a cross-sectional view of a monolithic barrier filmincorporating the present invention.

FIG. 17 is an isometric view of a cigarette package wrapped in a barrierfilm of the present invention shown in FIG. 15 and FIG. 16 and showingthe manner in which it overlaps to form good heat seals.

In general, the barrier film having a high colorless transparency iscomprised of a substrate formed essentially of a plastic having firstand second surfaces. A barrier layer is formed on the first surface andhas a thickness ranging from approximately 50 to 300 Angstroms and isformed from a material selected from the group of aluminum oxide(Al₂O₃), tin oxide (SnO₂) and yttrium oxide (Y₂O₃). An additional layerof silicon dioxide may be formed on the barrier layer and having athickness ranging from 100 to 1000 Angstroms.

More in particular, as shown in FIG. 1, the barrier film 11 is comprisedof a substrate 12 formed of a suitable flexible plastic as, for example,PET which is a polyester having a thickness ranging from approximately48 gauge which is approximately ½ mil to a thickness of 4 mils. Thesubstrate is provided with first and second surfaces 13 and 14. Anucleation layer 16 of a thickness ranging from approximately 50 to 100Angstroms is deposited on the surface 13. The nucleation layer is formedof a single material selected from pure aluminum oxide (Al₂O₃), pure tinoxide (SnO₂), pure yttrium oxide (Y₂O₃), with pure meaning 99.0% purityor better. Typically the nucleation layer is formed by electron beamevaporation of material onto the surface 13. If desired, the materialalso can be deposited by sputtering. Electron beam evaporation has beenfound to be desirable because it is possible to evaporate materialrapidly and over a wide area so that the material can be applied costeffectively on rapidly moving sheet material in a roll coater. By way ofexample, the material forming the nucleation layer can be evaporatedover a width of at least two meters and with a rapidly moving film.

A barrier layer 17 formed of silicon dioxide (SiO₂) is then depositedupon the nucleation layer 16. The starting material is silicon dioxidewhich is a clear transparent material. It is deposited by electron beamevaporation to a thickness ranging from 100 to 1000 Angstroms.Alternatively, if desired, the silicon dioxide can then be deposited byreactive sputtering. As hereinafter pointed out that MS-65 produced byFlex Products, Inc. of Santa Rosa, Calif. can be substituted for thesilicon dioxide.

The coating applied to the surface 13 which is comprised of thenucleation layer 16 and the barrier layer 17 is deposited in a two-stepprocess in which the nucleation layer 16 is deposited first followed bythe silicon dioxide barrier layer 17. This can be readily accomplishedon a roll coater by first depositing the nucleation layer 16 followed bythe silicon dioxide layer 17 at subsequent spaced apart locations in theroll coater. Alternatively, the two layers, the nucleation layer and thebarrier layer 17 can be deposited in two separate passes of the film inthe roll coater.

If still further improved barrier properties are desired for the film,the same type of coating which is placed on the one side or surface 13of the flexible plastic substrate 12 can also be placed on the otherside or surface 14 of the substrate 12. Thus, there has been provided inFIG. 2, a barrier film 18 which has the nucleation layer 16 and thebarrier layer 17 on the surface 13 and an additional nucleation layer 19on the surface 14 followed by a silicon dioxide barrier layer 21. Thetwo layers 19 and 21 can be formed of the same materials and can havethe same characteristics as the layers 16 and 17 on the surface 13 ofthe substrate 12.

When the barrier film of the present invention is to be utilized as apackaging material, a barrier film 24 of the type shown in FIG. 3 can beutilized. The embodiment of the barrier film 11 shown in FIG. 1 isprovided with a heat seal layer 26 formed of a suitable heat sealingmaterial well known to those skilled in the art. A suitable heat sealingmaterial would be cross-linked ethylene acrylic acid or high molecularweight ethylene acetate polymers or urethane polymers. Typically thiscould be accomplished by taking the roll coated polyester which has beencoated with the nucleation layer 16 and the barrier layer and depositingthe heat seal layer using a gravure coater or slot coater or otherappropriate coater. The heat sealing material typically would be of amaterial in which heat sealing could be accomplished at 100 to 150° C.with a few seconds of contact.

When heat seal capabilities are desired on both sides of a barrier film,a barrier film 27 such as that shown in FIG. 4 can be utilized in whichanother heat seal layer 28 is provided on the surface 14. This heat seallayer 28 can be formed of the same material as the heat seal layer 26.The heat seal layers can have a typical thickness ranging from 0.1-1mil.

Table 1 as set forth below which sets forth water vapor barrier filmdata for barrier films, on polyester terephthalate (PET) which has beencollected in making barrier films in accordance with the presentinvention.

TABLE 1 WATER VAPOR BARRIER FILM DATA WVTR Design Thickness Substrateg/100 sq.in./day BOX COATER UNCOATED 1 mil PET 1.2-1.3 UNCOATED 2 milPET .56-.63 2000 Å SiO₂ 2 mil PET 0.51 1000 Å SiO₂ 2 mil PET 0.67  500 ÅSiO₂ 2 mil PET 0.67  100 Å Al₂O₃/500 Å SiO₂ 1 mil PET 0.072  50 ÅAl₂O₃/500 Å SiO₂ 1 mil PET 0.063  25 Å Al₂O₃/500 Å SiO₂ 1 mil PET 0.419 10 Å Al₂O₃/500 Å SiO₂ 1 mil PET 1.39  100 Å Al₂O₃/500 Å SiO₂ 2 mil PET0.072  50 Å Al₂O₃/500 Å SiO₂ 2 mil PET 0.064  25 Å Al₂O₃/500 Å SiO₂ 2mil PET 0.159  10 Å Al₂O₃/500 Å SiO₂ 2 mil PET 0.68  100 Å Al₂O₃/250 ÅSiO₂ 1 mil PET 0.111  50 Å Al₂O₃/250 Å SiO₂ 1 mil PET 0.122  25 ÅAl₂O₃/250 Å SiO₂ 1 mil PET 0.14  10 Å Al₂O₃/250 Å SiO₂ 1 mil PET 1.45 100 Å Al₂O₃/250 Å SiO₂ 2 mil PET 0.104  50 Å Al₂O₃/250 Å SiO₂ 2 mil PET0.092  25 Å Al₂O₃/250 Å SiO₂ 2 mil PET 0.629  10 Å Al₂O₃/250 Å SiO₂ 2mil PET 0.69  100 Å SnO₂/500 Å SiO₂ 1 mil PET 0.058  50 Å SnO₂/500 ÅSiO₂ 1 mil PET 0.073  25 Å SnO₂/500 Å SiO₂ 1 mil PET 0.589  100 ÅSnO₂/500 Å SiO₂ 2 mil PET 0.073  50 Å SnO₂/500 Å SiO₂ 2 mil PET 0.079 25 Å SnO₂/500 Å SiO₂ 2 mil PET 0.364  100 Å Y₂O₃/500 Å SiO₂ 2 mil PET0.0468  50 Å Y₂O₃/500 Å SiO₂ 2 mil PET 0.0747  25 Å Y₂O₃/500 Å SiO₂ 2mil PET 0.26  100 Å Y₂O₃/500 Å SiO₂ 1 mil PET 0.035  50 Å Y₂O₃/500 ÅSiO₂ 1 mil PET 0.0917  25 Å Y₂O₃/500 Å SiO₂ 1 mil PET 0.542 ROLL COATER 75 Å Al₂O₃/250 Å SiO₂ 1 mil PET 0.14  75 Å Al₂O₃/375 Å SiO₂ 1 mil PET0.13  75 Å Al₂O₃/500 Å SiO₂ 1 mil PET 0.14

It can be seen that the data in the above-identified table is gatheredfrom two sources; a box coater and a roll coater. The water vaportransmission rate is set forth in grams per 100 square inches per day ofbarrier film. The flexible plastic substrate utilized had a thickness of1 and 2 mils as shown. The table shows when the substrate was uncoatedit had a water transmission rate appears to be directly related to thethickness of the film. Coating of the substrate with silicon dioxidealone at various thicknesses did very little, if any, to increase thebarrier properties of the film. However, the addition of a nucleationlayer formed of one of the materials previously identified gave dramaticimprovements in the reduction in water vapor transmission rate as shownin the above table by a factor of approximately 20. It also shows thatthe water vapor transmission rate was not changed significantly byincreasing the thickness of the substrate.

Certain other data shown in Table 1 has been plotted in the graphs shownin FIGS. 6, 7 and 8. The data for the aluminum oxide nucleation layer isplotted in FIG. 6, whereas the data for the tin oxide nucleation layeris plotted in FIG. 7. Four plots are shown in FIG. 6. From FIG. 6, itcan be seen that three variables are being plotted, the substratethickness, the Angstroms of aluminum oxide and the thickness of thesilicon dioxide barrier layer. The graph shows that the thicker layersof silicon dioxide are more effective as a barrier to water vaportransmission than the thinner layer.

In examining the chart in FIG. 6, it also can be seen that when thenucleation layer of aluminum oxide becomes less than 50 Angstroms, thewater vapor transmission rate rises very sharply so that the combinationof the aluminum oxide nucleation layer and the silicon dioxide layer isrelatively ineffective as a water vapor barrier. Thus it can be seenthat the basic parameter which controls the large variation in watervapor transmission rate is the thickness of the aluminum oxidenucleation layer. There is a minor change due to the thickness of thesilicon dioxide layer.

In comparing the results in box coaters with roll coaters as shown inTable 1, it can be seen that the results obtained in the roll coatershow that the water vapor transmission rate is not quite as low in theroll coater as in the box coater. This is believed to be due to the factthat there are more variables in a roll coater making it more difficultto achieve the same results as in a box coater. However, it can be seenthat the results are substantially equivalent and that the greatadvantage in using the roll coater in producing barrier films at highcapacity can make the use of roll coaters for forming such barrier filmsvery desirable because of the cost effectiveness of the roll coaters.

From examining the data which is shown in Table 1, it can be seen thatimprovements by a factor of 20 can be obtained in the water vaportransmission rate with the use of the nucleation layer and the silicondioxide layer of the type hereinbefore described. In addition, it can beseen that the water vapor transmission rate is relatively independent ofthe thickness of the substrate indicating that the barrier qualities areprincipally provided by the coating on the substrate.

Of the materials utilized for a nucleation layer, the aluminum oxide,the tin oxide and the yttrium oxide, all provide barrier films which arewater white transparent. When tin oxide is utilized as a nucleationlayer with silicon dioxide as shown in the graph in FIG. 7, resultssimilar to that shown in FIG. 6 are obtained. Again, it can be seen thatwhen the nucleation layer has a thickness of less than 50 Angstroms, thewater vapor transmission rate rises sharply so that it is apparent thatwhenever the nucleation layer is less than 50 Angstroms in thickness,the water vapor barrier capabilities rapidly diminish. It also can beseen that the thickness of the substrate has very little effect on thewater vapor transmission characteristics, although, it is noted that aslightly improved vapor transmission rate is achieved with the thickersubstrate.

When yttrium oxide is used as the nucleation layer with silicon dioxideas shown in the graph in FIG. 8 results similar to that obtained inFIGS. 6 and 7 are obtained. These graphs in FIGS. 6, 7 and 8 show thatwith all four designs with 1 and 2 mil PET, the nucleation layer has aneffect at 50 Angstroms and above in thickness.

It has been noted that sometimes when the heat seal coatings areapplied, the WVTR's are reduced even further. This effect is attributedto the heat seal coating filling up microscopic pores or pin holes inthe oxide coating. This is demonstrated by Table 2 set forth below whichgives water vapor transmission rates of barrier films of the presentinvention.

TABLE 2 WATER VAPOR TRANSMISSION RATES (WVTR) WVTR of Heat Sealed 1 milICI 393/Al₂O₃ 75 Å/SiO₂ 250 Å* Oxide Barrier  0.33 ± 0.026 Heat Seal/0.135 ± 0.034 Oxide Barrier *Heat seal thickness = 0.15 mil

An oxygen permeation test on the 1 mil of roll coated material is setforth below in Table 3:

TABLE 3 OXYGEN TRANSMISSION Design Thickness O₂ TR 75 Å Al₂O₃ 500 ÅSiO₂0.15 cc/100 sq.in./day 100 Å Y₂O₃ 500 ÅSiO₂ 0.0645 cc/100 sq.in/dayuncoated substrate 3-4 cc/100 sq.in/day

In accordance with the present invention, it has been found that singlelayers of very thin Al₂O₃ or Y₂O₃ in the range of 75 Å to 175 Å giveextremely good barriers toward water vapor as set forth in Table 4below.

TABLE 4 WATER VAPOR BARRIER DATA BOX COATER WVTR Design ThicknessSubstrate g/100 sq.in./day Uncoated 1 mil PET 1.2-1.3 Uncoated 2 mil PET.56-.63 100 Å SnO₂ 1 mil PET 1.06 100 Å SnO₂ 2 mil PET 0.56 100 Å Y₂O₃ 1mil PET 0.08 100 Å Y₂O₃ 2 mil PET 0.086 100 Å Al₂O₃ 1 mil PET 0.08 100 ÅAl₂O₃ 2 mil PET 0.091 100 Å MS65 1 mil PET 0.0461 100 Å MS65 2 mil PET0.051 100 Å SnO₂/500 Å SiO₂ 1 mil PET 0.073 100 Å SnO₂/500 Å SiO₂ 2 milPET 0.079 100 Å Y₂O₃/500 Å SiO₂ 1 mil PET 0.033 100 Å Y₂O₃/500 Å SiO₂ 2mil PET 0.075 100 Å Al₂O₃/500 Å SiO₂ 1 mil PET 0.072 100 Å Al₂O₃/500 ÅSiO₂ 2 mil PET 0.075 100 Å MS65/500 Å SiO₂ 1 mil PET 0.25 100 Å MS65/500Å SiO₂ 2 mil PET 0.18 100 Å Al₂O₃/500 Å MS65 1 mil PET 0.0375 100 ÅAl₂O₃/500 Å MS65 2 mil PET 0.05

It also has been found that even MS-65 produced by Flex Products, Inc.,a mixture of 65% SiO₂/35% MgO, as described in U.S. Pat. No. 4,702,963,gives exceedingly low WVTR's. This was totally unexpected becausepublished literature has indicated that 1000 Angstroms or greater inthickness of oxide coatings are necessary to achieve low WVTRs. Such abarrier film is shown in FIG. 5 in which a PET substrate 31 having asurface 32 is coated with a layer 33 of either yttrium oxide or aluminumoxide. A conventional heat seal layer 34 is provided. Alternatively,laminated polypropylene or polyethylene can be used for the heat seallayer 34.

It also has been found that low water vapor barrier properties can beachieved on roll coaters at speeds in excess of 100′ per minute andstill obtain results such as shown in Table 5 below. The roll coatedbarrier film appears to have a barrier property that is independent ofthickness.

TABLE 5 WVTR's on ICI-393, 1 mil PET WVTR Al₂O₃ Thickness g/100sq.in./day  75 Å 0.14 ± 0.03 100 Å 0.11 ± 0.02 150 Å 0.10 ± 0.01 200 Å0.12 ± 0.03

The foregoing establishes that Al₂O₃, SnO₂ and Y₂O₃ can be utilized as anucleation layer having a thickness ranging from approximately 50 to 100Angstroms and in combination with silicon dioxide providing asubstantial water vapor transmission reduction of approximately 20 fold.The thin nucleation layers above 50 Å and below 175 Å in thickness incombination with the silicon dioxide layer provides unexpected barrierproperties. In the case of the SnO₂ nucleation layer, one needs to usean SiO₂ layer on top of it because the SnO₂ by itself does not conferany reduction in water vapor transmission rates (see Table 4). Thesilicon dioxide overlayer is also beneficial in bonding to the heat seallayer and for providing abrasion resistance to the thin nucleationlayer.

In FIG. 9 there is shown a cross-sectional view of a pre-laminationconstruction 39 to be used in a packaging film. It consists of asubstrate 41 formed of a plastic which is capable of tearing in straightlines. A commercially available homo-polymer oriented polypropylene(OPP) film is utilized and has a thickness ranging from 55 gauge to 80gauge. In the simplest embodiment, the substrate can be formed of asingle layer of the homopolymer polypropylene. This is provided with anon-heat sealable characteristic in which only one surface is treatedwith a corona to provide an ionic discharge which raises the energylevel of the surface of the film and permits organic coatings to wetthat surface so that printing or graphics can be provided on thatsurface of the film.

In order to provide tearing along straight lines desired for thispackaging film, the homopolymer polypropylene is bi-axially oriented(OPP) so that it has fairly uniform mechanical characteristics in bothmachine and transverse directions of the film. Other commerciallyavailable polypropylene products include a layer of homopolymerpolypropylene which has co-extruded thereon a heat seal polymeric layerof a suitable thickness, such as 0.02 mils. A tri-layer design is alsocommercially available which has a layer of homopolymer polypropylenewith both sides having formed thereon co-extruded layers of a heat sealpolymeric layer of a suitable thickness, as for example, 0.02 mils. Ineither structure described above, one of the exterior surfaces can beprovided with a corona surface treatment of the type hereinbeforedescribed. Additional substrate types that are useful for thisapplication include either polymeric films that have been coated withheat sealable layers on one or both surfaces after extrusion.

The substrate 41 shown in FIG. 9 can take the form of any of the varioustypes of polypropylene films hereinbefore described. The barrier layercan be deposited on either side of the polypropylene but the preferredside is the one that has been corona treated. In the embodimentspreviously described, a nucleation layer has been provided followed by asilicon dioxide layer. In further work, it has been found that most ofthe barrier properties on PET were provided by the nucleation layer,except in the SnO₂ case rather than the SiO₂ layer. For that reason, thethickness of the nucleation layer was increased and the use of thesilicon dioxide layer was discontinued. The nucleation layer wastypically formed of a thickness ranging from 150 to 275 Angstromsdepending upon the ultimate barrier properties desired.

Table 6 below shows some WVTR results for roll coated polypropylene withvarious barrier layers with and without lamination.

TABLE 6 EFFECTS OF LAMINATION² ON BARRIER COATED WEBS BARRIER WEB ID WEBTYPE TYPE WVTR¹ 1313-3094 80 ga. OPP MS65 0.087 TM636-5 1313-3094/55 ga.OPP MS65 0.073 TM636-11 1313-3094/1313-3094 MS65 0.026 1313-3092 80 ga.PET AL₂O₃ 0.098 TM636-21 1313-3092/92 ga. PET AL₂O₃ 0.058 TM636-271313-3092/1313-3092 AL₂O₃ 0.030 ¹WVTR IN GM/100 SQ IN/24 HR ²LAMINATIONSWERE MADE BY HAND TM636-5 is a lamination of 1313-3094 and 55 ga. OPP.TM636-11 is a lamination of 1313-3094 to itself. TM363-21 is alamination of 1313-3092 to 92 ga. PET TM363-27 is a lamination of1313-3092 to itself.

The above Table 6 shows that by laminating the two barrier coated websinto a composite a synergistic affect is realized. A three-foldimprovement in water vapor transmission was realized where a two-foldimprovement would ordinarily be expected.

In order to provide heat sealable capabilities for a packaging film, alaminating sheet 46 formed of a polypropylene can be of the type inwhich there is provided a co-extruded heat sealable surface on one sideand a corona treated surface on the other side with the co-extruded sideproviding a surface 47 and the corona treated side providing a surface48. The surface 48 is provided with a suitable laminating adhesive 49 toprovide a laminating sheet construction 51. The two constructions 39 and51 are bonded together in the manner shown in FIG. 11 in which thelaminating adhesive 49 is brought into contact with a thin barriercoating 43 by the application of heat and pressure so that there is apackaging film 52 which is heat sealable on both sides by use of thesurfaces 44 and 47. Typically, the laminating operation is carried outby applying the adhesive as a wet coating and then heating the same,driving out the carrier solvents so that there remains a viscous tackyadhesive. This adhesive is similar to a pressure sensitive adhesivewhich is adhered to the thin film which, with the application of heatand pressure, causes the laminating adhesive to be activated and to sealto the barrier coating and to bond the two substrates 46 and 41 togetherso they cannot be peeled apart easily.

Typically, the laminating sheet 46 would have a thickness of ½ mil to ¾mil whereas the adhesive would have a thickness of approximately{fraction (1/10)}th of a mil. Typically, the substrate 41 can be formedof 70 gauge material whereas the laminating sheet 46 can be formed of 55gauge material. It is possible to achieve a laminated construction suchas that shown in FIG. 11 having a thickness of approximately 1.25 mils.

If it is desirable to have a thinner packaging film, an approach such asthat shown in FIG. 12 can be utilized because at the present timepolymeric substrates of thinner material that are heat sealable are notcommercially available. The packaging film construction shown in FIG. 12is comprised of a homopolymer polypropylene or PET substrate 57 havingtwo surfaces 58 and 59. A barrier coating 61 is applied to the surface58 in the manner hereinbefore described to a suitable thickness, as forexample, 150 Angstroms utilizing aluminum oxide. A heat seal coating 62is provided on the barrier film 61 to provide a heat seal coating whichcan be utilized on one side of the packaging film. Depending upon thematerial utilized for the substrate 57, if needed, a heat seal coating63 can be provided on the other side 59. Utilizing such a constructionit is possible to achieve a packaging film having an overall thicknessof less than 0.9 mil with the 70 gauge substrate 57 having a thicknessof 0.70 mil and with each of the heat seal coatings 62 and 63 having athickness of 0.1 mil.

In applications where the packaging film is subjected to heat andtension, it is preferable to utilize the construction shown in FIG. 11in which the laminating sheet 46 is provided to accommodate the heatingand tensioning which may take place. This ensures that the thin filmbarrier coating will not be deleteriously affected by the heating andtensioning of a heat seal coating. In the construction shown in FIG. 11,the packaging film can be provided without excessive heat or tensioningmerely by introducing the two substrates 39 and 51 into a laminating nipafter the laminated adhesive has been applied without subjecting thebarrier coating 43 to excessive heat. The only disadvantage is thatthere is an increased cost for the additional laminating sheet 46 andthe additional thickness which may make it necessary to run packagingmachinery utilizing the packaging material to run at a lower speedbecause of the increased heat required for heat sealing because of thethicker packaging material.

In FIG. 13 there is shown another construction of a barrier-typepackaging film 64 which is comprised of biaxially oriented coextrudedpolypropylene substrates 66 and 67 having surfaces 68 and 69respectively upon which there is provided barrier coating 71 and 72 of asuitable thickness ranging from 50 to 180 Angstroms formed of a suitablematerial such as aluminum oxide. The two substrates 66 and 67 arelaminated together by applying a laminating adhesive 73 between thefacing barrier films 71 and 72 so that the barrier films are face toface in the center of the laminated construction. Such a barrier typepackaging film has at least two advantages. By placing the two barriercoatings 71 and 72 facing each other any cracks or holes appearing inone of the barrier coatings would be covered by the other barriercoating to thus, in effect, provide double protection. By providing thebiaxially coextruded polypropylene substrates 66 and 67 on oppositesides, the barrier-type packaging film 66 can be heat sealed ontoitself.

In FIG. 14, a graph is shown in which the water vapor transmission datafor a single layer of aluminum oxide on 70 gauge polypropylene rangingin thickness from 25 to 300 Angstroms. The graph shows that the watervapor transmission decreases substantially from 25 Angstroms to 100Angstroms and then it approximately the same for greater thicknesses ofaluminum oxide.

In comparing the graph shown in FIG. 14 with the graphs shown in FIGS.6, 7 and 8, it can be seen that a greater thickness of aluminum oxide isrequired to achieve the same water vapor transmission rates. Thepolypropylene requires a greater thickness of barrier coating than doesthe polyester.

From the foregoing, it can be seen that when a barrier coated substrateis laminated to a heat sealable substrate and when a heat seal coatingis applied directly to a barrier film, a packaging film is created whichcan be used as an overwrap to retard the moisture vapor and gasmigration to or from foods, pharmaceutical devices, tobacco products andother industrial products.

As hereinbefore disclosed for polyester based barrier films, a heat sealcoating is required to provide a sealing surface. Heat seal coatings maybe applied to either/or both sides of the barrier coated film dependingon the particular finished product seal or laminating requirements. Forlap-type seals, the heat seal coating would normally be applied to bothsides of the barrier coated film. For fin-type sealing or lamination,the heat seal coating would normally be applied to the barrier coatedside only. For certain special applications, the heat seal coating canbe applied to the non-barrier coated side of the film.

For polypropylene films, a heat seal coating may or may not be requiredfor both surfaces of the film depending upon the polypropylene chosenand on the particular application.

For the heat seal layers, conventional heat seal coating resin systemscan be utilized which can include polyester urethane, ethyleneinterpolymers (EVA, EAA type polymers) and acrylic based formulas. Bothwater reduced and solvent reduced coatings can be used depending uponthe type of substrate chosen and the surface treatment of the substrate.In general it has been found that water based heat seal coatings requirea corona treated surface to obtain good adhesion of the coating to thesubstrate. These heat seal coatings can be modified with slip agents toimprove their machinability.

From the foregoing it can be seen that there has been provided a barrierfilm which is highly transparent in the visible region which serves as abarrier to water vapor and oxygen. Polymeric film substrates can beused. The use of the vacuum deposited inorganic oxide such as thesilicon dioxide makes it possible to replace existing aluminizedpolyester and co-extruded polymeric films. Because of the capabilitiesof utilizing thinner substrates, the total volume of plastic utilized isgreatly reduced. The use of the silicon dioxide or aluminum oxidecoating will also reduce the difficulty in recycling plastic because inrecycling processes, the thin oxide film will simply act as a tracecomponent and can be worked into the new polymer as a filler. Since theuse of a metal layer has been eliminated by the use of inorganic oxides,the barrier film can be utilized for food packaging which can be heatedin microwave units. The film of the present invention permits simplerpackaging while permitting the customers to view the contents before useand still provide a long shelf life. By utilizing the barrier coating inthe present barrier film, the use of PVC and PVDC as barrier materialsis eliminated overcoming the possibility of unreacted monomers reachingfood contents in the package. The barrier film in the present inventionis also advantageous in medical packaging because it permits viewing ofthe contents of the bag without opening of the same.

The barrier film is also particularly desirable for use in packagingmaterials where a predetermined moisture content must be maintained inthe product. Tobacco is an example of such a product in which it isdesired to provide a predetermined moisture content. If excessivemoisture is present, the product will mold. If insufficient moisture ispresent, the product tastes stale. The barrier film of the presentinvention makes it possible to package tobacco products in a clear filmand at the same time to provide a high shelf life in all kinds ofclimates including those that have high humidity as present in thetropics and those that have low humidities as in desert climates. Byutilizing biaxially oriented polypropylene it is possible to providebarrier films which will tear in straight lines making them particularlydesirable for use in packaging certain products as, for example,cigarettes.

By providing the barrier film with layers of heat sealing material, thematerial can be heat sealed onto itself to provide simplified packaging.By providing a heat sealing layer on both sides, the material can befolded either way to heat seal on itself. Also, in certain types ofpackaging, it is an advantage to provide heat sealing capabilities ofboth sides on the barrier film.

It should be appreciated that the nucleation layer and barrier layercombinations can be deposited by other methods of vacuum depositionincluding C.V.D. plasma-type sputtering processes, ion assistedprocessed such as ion plating, as for example, meta-mode (TM), as wellas others well known to those skilled in the art of vacuum deposition.

In connection with the embodiments of the barrier film incorporating thepresent invention hereinbefore described, difficulties have beenencountered and utilized in the same connection with cigarette packaginglines because the typical 130 gauge film produced could only be used onselected packaging lines. In order to produce a barrier film forpackaging of reduced thickness, as for example 90 to 80 gauge so that itcan be utilized in conventional cigarette packaging machines, themonolithic barrier film 81 shown in FIG. 15 has been provided. As showntherein it consists of a base film or substrate 82 which is providedwith first and second surfaces 83 and 84 disposed on opposite sidesthereof. The base film 82 can have a suitable thickness ranging from,for example 50 gauge to 70 gauge. It has been found that the base filmor substrate 82 can be formed of a polyester or alternativelypolypropylene (OPP). One polyester found to be satisfactory was theHoechst 2CXI that was made without its normal surface treatment. Thissubstrate has been given a tentative identification number of EH-1913.Another polyester found to be satisfactory was identified as PT4000supplied by Hoechst which was selected because its smooth surfaceproperties. A satisfactory polypropylene product was found to besupplied by Hercules and identified as BK-522 which can be characterizedas a composite of polypropylene and Surlyn. It is desirable that thematerial is utilized for the base film or substrate 82 have goodfriction characteristics, as for example the coefficient of frictionshould preferably be approximately 0.5 or less to facilitate the runningof the barrier films on packaging machines. A barrier layer or coating86 is formed on the surface 83 and a heat seal layer 87 is formed on theopposite surface 87. The barrier coating 86 is formed of materialshereinbefore described in connection with the previous embodiments, asfor example aluminum oxide. Typically the surface 83 in which thebarrier coating 86 is deposited is untreated. The heat seal layer 87 issupplied by Hoechst as a part of its 2CX1 product and typically can beformed of a suitable material such as a lower melting point polyesterand had a thickness ranging from 10 gauge to 30 gauge with the polyesterbase film or substrate 82 having a thickness ranging from 40 to 70gauge. When the base film or substrate was a composite such aspolypropylene and Surlyn, the heat seal 87 was in the form of a Surlynheat seal material supplied by Hercules as a part of its BK-522 product.To prevent blocking (i.e. sticking) of the barrier film, the Surlynlayer was modified using a Surlyn with a higher tack temperature. ThisOPP/Surlyn substrate is identified as BK-522XQ79. The heat seal layer isformed of polar materials to facilitate the formation of bonds to highsurface energy surfaces, such as oxides, to itself, or to polarpolymeric films.

In accordance with the present invention it has been found that thebarrier coating of the present invention can be satisfactorily appliedover a heat seal as shown in FIG. 16. Barrier film 91 has a heat seal 87of the type hereinbefore described formed on both of the surfaces 83 and84 and then has the barrier coating 86 formed on the surface of the heatseal 87.

In connection with utilizing the barrier film shown in FIGS. 15 and 16,as for example the packaging of cigarettes, it is important that thebarrier layer faces the package so that it will not be abraded ordamaged during handling or by being exposed directly to the elements. Toaccomplish this, it is desirable that the barrier layer face the packageor carton while at the same time to make it possible to obtain totalsealability of the package to be sure that it is air-tight. Such apackaging arrangement is shown in FIG. 17. The package 96 shown in FIG.17 is in the form of a parallelepiped representing a conventionalcigarette package configuration in which the barrier film is wrappedabout the inner package 97 and sealed to provide air-tight heat sealsbetween the film overlaps. The inner package 97 can be formed of aconventional material such as cardboard or paper which typically issusceptible to oxygen and water vapor penetration. The manner in whichthis is accomplished is shown in FIG. 17 wherein the overlap areas havebeen designated with the letters A, B, C, D, E and F. The manner inwhich the barrier coating faces in toward the package and in which theheat seals are made between the overlaps is shown in the diagrams setforth below.

OVERLAP A Heat Seal Base Film Barrier Coating PACKAGE OVERLAP B HeatSeal Base Film Barrier Coating Heat Seal Base Film Barrier CoatingPACKAGE OVERLAP C Heat Seal Base Film Barrier Coating Barrier CoatingBase Film Heat Seal Heat Seal Base Film Barrier Coating PACKAGE OVERLAPD Heat Seal Base Film Barrier Coating Barrier Coating Base Film HeatSeal Heat Seal Base Film Barrier Coating Heat Seal Base Film BarrierCoating PACKAGE OVERLAP E Heat Seal Base Film Barrier Coating BarrierCoating Base Film Heat Seal Heat Seal Base Film Barrier Coating BarrierCoating Base Film Heat Seal Heat Seal Base Film Barrier Coating PACKAGEOVERLAP F Heat Seal Base Film Barrier Coating Barrier Coating Base FilmHeat Seal Heat Seal Base Film Barrier Coating Barrier Coating Base FilmHeat Seal Heat Seal Base Film Barrier Coating Heat Seal Base FilmBarrier Coating PACKAGE

It can be seen from above that the barrier coating always faces inwardlyof the package and that the heat seals are formed between a heat seallayer and a barrier layer or coating. Thus, with the barrier films 81and 91 shown in FIGS. 15 and 16 it is possible to provide packages witha barrier film having the barrier coating facing inwardly of the packageso that it is not subject to abrasion and damage and therefore remainsintact.

It has been found that the barrier films 81 and 91 shown in FIGS. 15 and16 have very desirable properties. The barrier films have a thicknesswhich is approximately 80 gauge or less so that it can be readily run onconventional cigarette packaging machinery. The coefficient of frictionis low, approximately 0.5 or less. The water vapor transmission rate(WVTR) range is from 0.06-0.12 grams per 100 square inches per day andthe oxygen transmission rate range is from 0.9 to 50 cubic centimetersper 100 square inches per day. Specific examples of these transmissionrates for uncoated and coated materials are set forth below.

uncoated coated O₂TR (cc/100 in²/day) Hercules BK-522 (OPP/Surlyn) 19349.17 Hoechst EH-1913 4.78 0.918 WVTR (gms/100 in²/day) Hercules BK-522(OPP/surlyn) 0.6620 0.1028 Hoechst 2CX1 PET 2.0600 0.0661

From the foregoing it can be seen that excellent water vapor and oxygentransmission rates can be achieved with a monolithic barrier film ofreduced thickness permitting it to be used on conventional packagingmachinery.

It should be appreciated that although considerable emphasis has beenplaced on the packaging of cigarettes with the barrier films of thepresent invention, the barrier films have many other applications in thefood and medical industries, as for example in the food where flavorretention is necessary.

What is claimed is:
 1. A method for forming a barrier film having a highcolorless water white transparency comprising the steps of providing aflexible plastic substrate having a surface and evaporating a singledielectric material having a purity of 99.0% or greater selected fromthe group consisting of aluminum oxide, tin oxide and yttrium oxide toform a barrier layer of such material having a thickness of 50 to lessthan 300 Angstroms directly onto said surface of said plastic substratefor reducing water and oxygen permeability.
 2. A method as in claim 1together with the step of forming a heat seal layer on the barrierlayer.
 3. A method as in claim 1 wherein the material utilized for thebarrier layer is evaporated by electron beam evaporation.
 4. A methodfor forming a barrier film having a high colorless water whitetransparency comprising the steps of providing a flexible plasticsubstrate having a surface and evaporating a single dielectric materialselected from the group consisting of aluminum oxide, tin oxide andyttrium oxide to form a barrier layer of a thickness of 50 to less than300 Angstroms directly onto said surface of said plastic substrate forreducing water and oxygen permeability, said barrier layer being formedby reactive sputtering in a vacuum.
 5. A method for forming a barrierfilm having a high colorless water white transparency comprising thesteps of providing a flexible plastic substrate having a surface andevaporating a single dielectric material selected from the groupconsisting of aluminum oxide, tin oxide and yttrium oxide to form abarrier layer of a thickness of 50 to less than 300 Angstroms directlyonto said surface of said plastic substrate for reducing water andoxygen permeability, providing an additional flexible plastic substratehaving a surface, providing a laminating adhesive on the surface of theadditional substrate and laminating the additional substrate to thefirst named substrate by bringing the laminating adhesive into contactwith the barrier layer.
 6. A method for forming a barrier film having ahigh colorless water white transparency comprising the steps ofproviding a flexible plastic substrate having a surface and evaporatinga single dielectric material selected from the group consisting ofaluminum oxide, tin oxide and yttrium oxide to form a barrier layer of athickness of 50 to less than 300 Angstroms directly onto said surface ofsaid plastic substrate for reducing water and oxygen permeability andproviding a heat seal coating on the barrier layer and on the side ofthe substrate opposite the barrier layer.
 7. A method for forming abarrier film having a high colorless water white transparency comprisingthe steps of providing a flexible plastic substrate consistingessentially of polypropylene and having a surface and evaporating asingle dielectric material selected from the group consisting ofaluminum oxide, tin oxide and yttrium oxide to form a barrier layer of athickness of 50 to less than 300 Angstroms directly onto said surface ofsaid flexible plastic substrate for reducing water and oxygenpermeability, providing an additional flexible plastic substrate formedessentially of polypropylene, providing a barrier layer on theadditional flexible plastic substrate and providing a laminatingadhesive between the barrier layer on the first named flexible plasticsubstrate and the barrier layer on the additional substrate.
 8. A methodfor forming a barrier film having a high colorless water whitetransparency comprising the steps of providing a flexible plasticsubstrate having a surface and evaporating a single dielectric materialselected from the group consisting of aluminum oxide, tin oxide andyttrium oxide to form a barrier layer of a thickness of 50 to less than300 Angstroms directly onto said surface of said plastic substrate forreducing water and oxygen permeability and evaporating onto the barrierlayer a layer of silicon dioxide having a thickness ranging from 100 to1000 Angstroms.
 9. A method as in claim 8 wherein the barrier andsilicon dioxide layers are evaporated onto the substrate while thesubstrate is moving.
 10. A method as in claim 8 wherein the materialutilized for the silicon dioxide layer is evaporated by utilizingelectron beam evaporation.
 11. A method as in claim 8 wherein thesilicon dioxide layer is reactively sputtered onto the barrier layer.